The present invention is in the field of semiconductor manufacturing and it specifically relates to devices and methods for protecting the front side of device wafers during grinding, polishing, or chemical mechanical planarizing of very thin and frangible wafers.
As described in U.S. Pat. No. 5,476,566 issued Dec. 19, 1995 to Cavasin, semiconductor wafers are typically sawn to approximately a 30 mil (0.76 millimeter) thickness as they are being manufactured. Thereafter, electronic circuitry is formed on one side of the wafer, called the front or active side. The front side of the wafer is planarized at various stages of electronic circuit formation, usually by a process known as chemical mechanical planarization (CMP). The wafers may then be thinned in subsequent processing by a backgrinding process wherein the backside, i.e., the side not covered by circuit components, is ground until the wafer is of a reduced thickness, typically on the order of 14 mils (0.36 millimeter).
Advances in device packaging have led to a demand for very thin wafers, on the order of 8 mil (0.20 millimeter) in thickness.
Unfortunately, the ongoing demand for thinner wafers has been accompanied by an ongoing demand for larger-diameter wafers. Wafers having a nominal diameter of 8.0 inches (actually 200 millimeters) are replacing wafers nominally 6.0 inches (actually 150 millimeters) in diameter. As a result of these converging trends, breakage of the wafers during processing has become a serious problem. If the breakage occurs after a number of process steps have been performed on the wafer, the result can be costly, thereby undermining the benefits gained by using larger diameter wafers. Thus, the forces applied to the wafers in the backgrinding process must be applied uniformly across the wafer to minimize breakage.
The problem is further aggravated by the delicate nature of the circuitry built up on the active front surface of the wafer, which ideally should not be scratched by sliding under the pressure applied by a chuck during the backgrinding process. To protect the wafer against this eventuality, it is known in the art (U.S. Pat. No. 5,476,566) to apply an adhesive tape to the front surface of the wafer to protect the circuitry while the backside of the wafer is being ground to achieve a reduced thickness.
In U.S. Pat. No. 5,964,646 issued Oct. 12, 1999 to Kassir et al. there is described the advantageous use of a resilient pad between the wafer and the chuck that supports the wafer during the grinding process. The use of the resilient pad is found to improve the flatness of the ground surface.
The present invention builds on this earlier work by providing a tool that facilitates installing a resilient tape on the chuck, rather than on the front side of the wafer. This permits the resilient tape to remain in place while a number of wafers are ground, polished, or planarized in succession, and this is much more efficient than the prior art technique of applying a resilient tape to each wafer.
Several circumstances complicate the apparently simple operation of applying an adhesive tape to the chuck that is used to hold the wafer during grinding, polishing, or planarization.
First, a widely used type of chuck is composed of an open cell porous ceramic, which allows the wafer to be held against the chuck by a vacuum applied through the ceramic material. As grinding progresses, grinding dust tends to be pulled into the pores of the ceramic chuck. These dust particles can come into contact with the device side of subsequent wafers, causing damage. Therefore, to protect wafers later processed on the same chuck, the dust is usually removed from the chuck after each wafer has been ground. The dust is typically removed by backflushing, whereby water is forced through the porous ceramic chuck in a direction opposite the vacuum airflow. Backflushing would tend to remove the tape from the chuck, but if the adhesion of the tape were great enough to withstand the water pressure, then backflushing could not take place.
A second problem encountered in applying a tape to the chuck is that the tape would tend to seal the ceramic material, defeating the vacuum that is relied on for holding the wafer to the chuck.
A third problem that the present inventors needed to solve was preventing air from becoming trapped between the tape and the chuck. Air bubbles trapped between the tape and the chuck caused the tape to be not completely flush against the chuck, and so the tape was not uniformly flat. Thus, the wafer resting on the tape would be more prone to defects after backgrinding, polishing, or CMP processes were performed.
The approach taken by the present inventors is to replace the porous ceramic chuck with a solid chuck having passages through it to permit the vacuum to be applied to the wafer. They then use a tape having apertures that register with the ends of the passages at the face of the chuck. Finally, to prevent air from being trapped between the tape and the solid chuck, the tape is applied to the chuck in a vacuum. To facilitate applying the tape to the chuck, the inventors developed a specialized tool that permits the operation to be accomplished swiftly and flawlessly.
In accordance with a preferred embodiment of the invention, a removable vacuum chamber fits over and sealingly engages the chuck. The removable vacuum chamber is indexed to the chuck by means of locating pins. Within the removable vacuum chamber a stamp is provided that is movable in a direction perpendicular to the face of the chuck. The tape to be applied is indexed to the face of the stamp, and then the stamp is advanced toward the chuck to bring the tape into adhesive contact with the face of the chuck. The absence of air within the chamber eliminates the possibility of air being trapped between the tape and the face of the chuck. After the tape has been applied to the chuck, the stamp is retracted, the vacuum within the chamber is relieved, and the chamber is removed from the chuck, leaving the chuck ready to receive a disc.
The novel features which are believed to be characteristic of the invention, both as to its structure and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in which several embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.